In steam turbines such as a low-pressure turbine, a vibrating stress may occur at a rotor blade under operating conditions with a low volume flow rate of main steam (in no-load operation at startup, in a low load operation, in operation under a low vacuum and so on). In particular, the rotor blade constituting a turbine stage at the final stage among a plurality of turbine stages has a large blade length, and therefore a large vibrating stress may occur thereat. This phenomenon occurs due to occurrence of a fluid exciting force in a flow field where the volume flow rate of main steam is low.
FIG. 9 is a view illustrating a part of a steam turbine. FIG. 9 schematically illustrates, for example, a part of a low-pressure turbine into which steam flows as a working fluid sequentially via a high-pressure turbine and an intermediate-pressure turbine. Additionally, in FIG. 9, flow fields occurring under the condition that the volume flow rate of steam is low are indicated by broken lines. In FIG. 9, the left side is the upstream side and the right side is the downstream side.
When the steam flows under the condition that the volume flow rate is low, reverse flow areas 203 occur near a rotor blade 202 of the turbine stage at the final stage composed of a stationary blade 201 and the rotor blade 202 as illustrated in FIG. 9. In the flow field where the reverse flow area 203 occurs, the flow becomes an unsteady state due to the rotation of the rotor blade 202, so that a fluid exciting force occurs at the rotor blade 202. In particular, a large fluid exciting force occurs near the tip of the rotor blade 202 and a large bending moment acts on the rotor blade 202, so that the vibrating stress becomes extremely large. An upper limit value σ1 of the vibrating stress is prescribed in consideration of characteristics such as a fatigue limit of a material, a safety factor and so on. Therefore, the operating range of the steam turbine is limited to conditions under which the vibrating stress does not exceed the upper limit value σ1.
FIG. 10 is a chart representing the relationship (vibrating stress characteristics) between the volume flow rate of the main steam and the vibrating stress of the rotor blade. In FIG. 10, the horizontal axis indicates the volume flow rate V of the main steam and a vertical axis indicates the vibrating stress σ.
As illustrated in FIG. 10, when the volume flow rate V is a predetermined rate V1, the vibrating stress σ becomes the upper limit value σ1. Therefore, when the volume flow rate V is lower than the predetermined rate V1, the vibrating stress σ includes a part exceeding the upper limit value σ1, and therefore the operating range of the steam turbine is limited to prevent the volume flow rate V from becoming lower than the predetermined rate V1. In other words, in the low load operation in which the flow rate of the main steam becomes low or the operation under a low vacuum in which a specific volume of the main steam becomes small, the load condition under which the operation is possible or the range of steam condition is limited.
In order to suppress occurrence of the vibration on the rotor blade at the final stage, various techniques have been proposed.
For example, it has been proposed that a plurality of rotor blades are coupled together to suppress the occurrence of the vibration. However, in this case, the range of the vibrating stress which can be suppressed is narrow, and it is not easy to sufficiently suppress the occurrence of the vibration in some cases.
Further, for example, it has been proposed that a part of steam discharged from the high-pressure turbine is inserted to the vicinity of the final stage of the low-pressure turbine via a by-pass line bypassing the high-pressure turbine and the intermediate-pressure turbine, to suppress the occurrence of the vibration. Besides, for example, it has been proposed that steam is inserted from the outside into the low-pressure turbine via a hollow part formed in a diaphragm outer ring, to suppress the occurrence of the vibration. However, in this case, there are many restraints such as an auxiliary boiler being required to supply the steam from the outside, a supply pipe system becoming large in scale, and a device being required to adjust the state of the steam to be inserted and so on. Therefore, the cost increases and the operation is not easy in some cases.
In addition to the above, under the condition that the volume flow rate of steam flowing through the turbine final stage is extremely low (at startup and so on), the rotation of the rotor blade at the final stage with a large blade length gives energy to the surroundings of the rotor blade, so that the temperature significantly increases. Therefore, the material strength of the rotor blade decreases and the thermal extension of the rotor blade may occur. For the countermeasures, it is generally performed to spray pure water to a turbine exhaust chamber to cool it. Sprayed spray water (liquid droplet) moves by the flow of the reverse flow area from the base side to the tip side of the rotor blade at the final stage to cool the rotor blade. On the other hand, spray water that has not evaporated due to heat exchange collides with the tip of the rotor blade at high speed in a liquid droplet state, so that damage may occur due to erosion.
In the above technique, under the condition that the volume flow rate of steam is low, it is not easy to sufficiently suppress the vibrating stress at the rotor blade in some cases. Further, it may be difficult to effectively suppress the temperature increase of the rotor blade and to sufficiently prevent erosion of the rotor blade and so on. As a result, it is not easy to widen the range of the operational steam flow rate in the steam turbine. In particular, the above-described problems become obvious in the rotor blade at the final stage with a large blade length, so that the conditions such as startup condition, load range, and vacuum degree condition are limited in some cases.
The problem to be solved by the present invention is to provide a steam turbine capable of easily widening the range of the operational steam flow rate and so on.